Recently, lower power consumption in a large scale integrated circuit (LSI) has been demanded. Especially, an LSI for a personal digital assistance (PDA), for example, has been set strict specifications. Therefore, lower power consumption has been studied as an important problem to be solved in LSI development.
Lowering power-supply voltage in LSI is significantly effective as a solution. On the other hand, an operation speed of a transistor is decreased when the power-supply voltage is lowered. As a result, a delay period for signal transmittance between circuits is increased. The signal transmittance of a critical pass which has the least allowance to signal delay may be decreased below a low limitation.
In such a situation, a method for setting a replica circuit which duplicates electrical characteristics of the critical pass, has conventionally proposed. In such a manner, the power-supply voltage of LSI is controlled corresponding to a judgment on characteristics of the replica circuit.
However, LSI in which variety kinds of functional blocks are mounted, has different kinds of combinations between functional blocks dependent on an operation mode. As a result, an amount of electrical current is different between one area and another area, for example, in one chip, where one area includes a function block which is just operating and another area includes a function block which is just stopping the operation.
Accordingly, a case is arisen where junction temperature of the transistor is different in the one chip. In an area where the junction temperature is increased, speed of the transistor is lowered.
Furthermore, lowering of the power-supply voltage is also generated due to a wiring resistance of a power-supply line in the area where the junction current is increased.
In other word, non-uniformity of the operation speed of the transistor is caused by a different operation mode.
Therefore, the conventional replica circuit may not necessarily duplicate the electrical characteristics of the critical pass due to a position of the replica circuit.
The situation mentioned above cause a problem that the power-supply voltage of the LSI may be not suitably set.